SPIRAL NEBULA WIRTZ 143 



and active part in onr world picture. To be more definite, let 

 us suj^pose this absorption to be of the magnitude produced by that 

 of a thick meteorological fog. Then light coming from beyond a 

 certain distance will be completely extinguished. We could receive 

 no information from more distant bodies. Our universe would have 

 the following appearance : Matter would appear gathered in a spher- 

 ical star system; our point of outlook would be at its center. It is 

 perhaps disquieting when we remember that already humanity has 

 had to look at the universe in a new way. That was when it threw 

 away the geocentric conception which was a quasi realization of such 

 an absorption cosmos. 



Some remarkable results have emerged from measures on the 

 absorption of light in space in which the stars are imbedded. In 

 numerous regions of the sky we see visually and on photographic 

 plates what may be called cosmic clouds. In other places stellar 

 counts reveal the existence of compact impenetrable masses of dust 

 situated at great distances and covering large tracts of the sky. 

 In the color of the stars a selective absorption is apparent, for the 

 more distant stars are redder. This property of absorption shows in 

 a remarkable way — stars in the richer regions are on the whole bluer 

 than where the stars are few. We conclude, ceteris paHhus, that the 

 regions poor in stars are poor because the more distant stars are 

 hidden behind cloudy veils of small particles which we know selec- 

 tively transmit red light. 



Likewise we encounter in all parts of the milky way traces of 

 both general and selective absorption. However, when we turn to 

 those parts of the sky where we suppose we see great distances out 

 into world space, all traces of both general and selective absorption 

 disappear. This is indicated by the globular star clusters which are 

 placed at some 10,000 to 300,000 light-years away. ' 



7. Can we obtain any evidence of absorption from the spiral nebu- 

 lae, the farthest of which may be situated millions of light-years dis- 

 tant ? Apparently we can. In them we have surface pictures of great 

 diameter. The surface brightness of these objects can be determined 

 photometrically. This is a very important datum, for the surface 

 brightness should be independent of the distance if space is free of 

 absorption — that is, optically empty. If, instead of taking as our 

 measure of their individually unknown linear distances, we take 

 their apparent diameter, the surface brightness determined for a 

 great number of these bodies should show in a statistical mean no 

 relation to their apparent diameter. We have at our disposal the 

 surface brightness of more than 500 spirals. We find, in submitting 

 all these measures to the precise methods of statistical mathematics, 



